A touch panel including not only a function to display an image, but also a function to detect a position touched by an object (a pen for input or the like) has been in wide use.
There are conventional touch panels made of two transparent substrates disposed such that transparent electrodes face each other through a spacer of several microns. In this touch panel, sensors connected to these transparent electrodes detect changes in resistance value or electric capacitance at a point where the transparent electrodes, which the two transparent substrates respectively have, touch each other, to detect a position touched by an object. However, because the transparent substrates are disposed slightly far from the display surface, there are problems that a user feels discomfort when operating, and the display quality is significantly lowered due to a generation of unnecessary reflected light.
In light of such problems, as the performance of elements constituting a TFT liquid crystal display has improved in recent years, TFT liquid crystal displays with a touch panel function in which a sensor is incorporated in respective pixels has been developed (for example, see Patent Document 1). This display enables a user to directly touch an image display surface because optical sensor elements are disposed next to TFT elements, and therefore, a user can specify a position without a feeling of discomfort. In addition, because this display does not have a structure in which transparent substrates are laminated, no unnecessary reflected light is generated, and an image can be displayed vividly.
In this TFT liquid crystal display, the optical sensor elements detect a position touched by a user. In this method, there are a method of detecting the shadow of an object such as a pen or a finger created by surrounding light, and a method of detecting light, which is transmitted light emitted from a backlight of the liquid crystal panel reflected on an object.
In TFT liquid crystal displays using the former method, the sensor elements could detect the shadow of an object just by getting the object closer to the display surface, causing a problem of an unintended input by a user. On the other hand, TFT liquid crystal displays using the latter method have a characteristic that the intensity of reflected light, which is detected by optical sensor elements, prominently increases when an object touches the display surface, and therefore, the above-mentioned problem is unlikely to occur.
Due to such a characteristic, various devices using the optical sensor element of the latter method as an area sensor (for example, an optical pointing input device in Patent Document 2) have been developed, besides the liquid crystal display unit described above.